Article coated with fluorocarbon polymer primer and fluorocarbon polymer topcoat

ABSTRACT

ARTICLES PRIMED WITH COMPOSITIONS CONTAINING 10-75% LITHIUM POLYSILICATE, 25-90% OF A FLUOROCARBON POLYMER AND A LIQUID CARRIER TO IMPROVE ADHESIVE OF A FLUOROPOLYMER TOPCOAT.

United States Patent O 3,692,558 ARTICLE COATED WITH FLUOROCARBONPOLYMER PRIMER AND FLUOROCAR- BON POLYh IER TOPCOAT Ervin R. Werner,Jr., Levittown, Pa., assignor to E. I. du Pont de Nemours and Company,Wilmington, Del. No Drawing. Continuation-impart of application Ser. No.

45,944, June 12, 1970, which is a division of application Ser. No.778,828, Oct. 25, 1968. This application Dec. 22, 1970, Ser. No. 100,765

Int. Cl. B321 15/08; B4441 N36 US. Cl. 117-72 12 Claims ABSTRACT OF THEDISCLOSURE Articles primed with compositions containing -75% lithiumpolysilicate, 25-90% of a fluorocarbon polymer and a liquid carrier toimprove adhesive of a fluoropolymer topcoat.

CROSS-REFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of application Ser. No. 45,944, filed June 12,1970, which in turn is a divisional of application Ser. No. 778,828,filed Nov. 25, 1968, both now abandoned.

BACKGROUND OF THE INVENTION This invention relates to coatingcompositions. It is n more particularly directed to primer compositionsfor improving the adhesion of fluorocarbon polymer coatings to theirsubstrates.

In recent years, use of fluorocarbon polymers as lubricious coatings hasbecome widespread. Cookware coated with polytetrafluoroethylene hasbecome especially popular because of its non-stock character. Thephysical nature of these fluorocarbon polymers makes it difiicult tobond them well enough to their substrates to prevent the coatings frompeeling away or blistering during use. This is especially true ofpolytetrafluoroethylene.

SUMMARY OF THE INVENTION I have found that adhesion of a fluorocarbonpolymer coating to its substrate can be significantly improved if acomposition of (a) particulate fluorocarbon polymer, (b) lithiumpolysilicate, and (c) a liquid carrier is used to prime the substratebefore the fluorocarbon polymer topcoat is applied.

The lithium polysilicate I use in my compositions is an aqueous lithiumpolysilicate containing about 15% through about 35%, by weight, ofsilica as SiO It has a SiO /Li O mol ratio of about 4-25/ 1 and is freeof alkali metals, other than lithium, to the extent that only thosealkali metals which cannot be removed by ion-exchange are present.

The lithium polysilicate preferred for use in my compositions containsabout through about by weight, of silica as SiO and has a SiO /Li O molratio of 4-10/ 1.

Especially preferred is the lithium polysilicate which contains about20% through about 25%, by weight, of silica as SiO and has a SiO /Li Omol ratio of 45.5/ 1.

The lithium polysilicate even more preferred contains about 20% throughabout 25%, by weight, of silica as SiO and has a SiO /Li O mol ratio of8-9/1.

Methods for preparing the aqueous lithium polysilicates suitable for usein my compositions are described in US.

Pat. No. 3,455,709. The portions of that patent which describepreparation of lithium polysilicates are incorporated into thisapplication by reference.

The lithium polysilicate is ordinarily present in my compositions at aconcentration of about 10% through about by weight of the total ofpolysilicate and fluorocarbon polymer solids, preferably about 15%through about 40%. The especially preferred compositions contain about25% of polysilicate.

The fluorocarbon polymer I use is particulate. The are those ofhydrocarbon monomers, preferably ethylenically unsaturated, completelysubstituted with fluorine atoms or a combination of fluorine atoms andchlorine atoms. Included in this group are polytetrafluoroethylene,copolymers of tetrafluoroethylene and hexafluoropropylene in all monomerunit ratios, and fluorochlorocarbon polymers such aspolymonochlorotrifiuoroethylene.

The fluorocarbon polymer I use is particulate. The particles should besmall enough to pass through the nozzle of a spray gun without cloggingit and small enough to give the resulting film integrity. In ordinarysituations, the particle should be no larger than about 0.35 micron inthe longest dimension.

Although I can use a dry flour or powder of fluorocarbon polymer andprovide a liquid carrier separately, I prefer to use the polymer in theform of an aqueous dispersion because it is most easily obtained on themarket in that form. Dispersions of fluorocarbon polymer in organicliquids miscible with water, such as ethanol, isopropanol, acetone andcellosolves, can also be used. In either case, the liquid also serves asa carrier for my composition.

The fluorocarbon polymer is ordinarily present in my compositions at aconcentration of about 25% through about 90%, by weight of the total ofpolysilicate and fluorocarbon polymer solids, preferably about 60%through about The especially preferred compositions contain about 75% offluorocarbon polymer.

My compositions are made by mixing proper amounts of an aqueous lithiumpolysilicate and a fluorocarbon polymer dispersion. The compositions canbe pigmented by preparing a suitable pigment dispersion by conventionaltechniques and then simply adding this pigment dispersion to thecomposition. The total solids content of these compositions can be35-45% by weight.

The resulting primer composition is applied in the usual way. If thesurface to be coated is metal, it is preferably pretreated bygrit-blasting, by the flame spraying v of metals or metal oxides, byfrit-coating the substrate, or by a combination of these methods.

After application, the primer coat is air-dried and then baked at230-400 C. for 10 to 20 minutes.

This primer coat is then topcoated with a conventional clear orpigmented fluorocarbon polymer enamel, and baked in the usual way, togive an adherent fluoropolymer coating.

Although I get the best coatings with the two-coat system justdescribed, I also get strongly adherent fluorocarbon polymer coatingswith a one-coat operation. To do this, I apply a thicker coat of theprimer composition, which may contain such pigments and surfactants asare necessary; this coat is then dried and baked as in the twocoatsystem. The advantage of this one-coat system in terms of labor cost isobvious.

My compositions are most useful for priming metal cookware, especiallyfrypans, for cOating with polytetrafluoroethylene, but they can also beused to prime other articles requiring fluorocarbon polymer filmspermanently bonded to their substrates. These articles can be made ofglass or other materials that can withstand the baking temperaturesused. For example, my compositions can be used to prime or coatbearings, valves, wire, metal foil,

boilers, pipes, ship bottoms, oven liners, iron sole-plates, waflieirons, ice cube trays, snow shovels and plows, chutes, conveyors, dies,tools such as saws, files and drills, hoppers, and other industrialcontainers and molds.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Those skilled in this art willbe able to practice my invention more easily by first referring to thefollowing illustrative examples. In these examples, all parts are byweight.

Example 1 A primer composition was prepared by thoroughly mixing andthen ball-milling the mixture for 16 hours.

An aluminum frypan was wiped with acetone to remove grease and then gritblasted. The pan was then cleaned of grit particles with a blast ofcompressed air and the primer composition sprayed on the pan to athickness of 0.20.4 mil (dry).

The pan was then baked for 20 minutes at 230 C. after which it wastopcoated with an enamel having the composition PartsPolytetrafiuoroethylene dispersion, aqueous, 50%

solids 99.5 Sodium salt of the sulfuric acid ester of a mixture of longchain alcohols (predominantly lauryl alcohol 0.5

by spraying the enamel over the primer to a thickness of 0.6-0.8 mil(dry) and then baking this at 400 C. for 20 minutes.

The resulting coating of polytetrafluoroethylene adhered well to thefrypan, with no blistering or peeling after extended use in the kitchen.

Example 2 A primer composition was prepared by thoroughly mixing PartsPolytetrafluoroethylene dispersion, aqueous, particle aqueous, particlesize about 0.2 micron average (longest dimension) 60% solids 375 Lithiumpolysilicate 85 375 Pigment dispersion 65 Sodium lauryl sulfonate 7.5

The pigment dispersion was made as shown in Example 1.

An aluminum frypan was then treated and coated with this primercomposition and topcoated, as shown in Example 1, to givea coating ofpolytetrafluoroethylene which adhered Well to the frypan, with noblistering or peeling after extended use.

Example 3 A primer composition was prepared by thoroughly mixing PartsPolytetrafiuoroethylene dispersion, aqueous, particle size about 0.2micron average (longest dimension),

and then pebble-milling the mixture for 16 hours.

An aluminum fry pan was grit-blasted and flit-coated. The primercomposition was sprayed on the pan to a thickness of 0.2-0.4 mil (dry).The pan was baked for 20 minutes at 230 C. and then topcoated, as shownin Example 1, to give a coating of polytetrafluoroethylene which adheredwell to the fry pan, with no blistering or peeling after extended use.

I claim:

1. An article bearing a fused film of a composition consistingessentially of (a) about 10%through about 75%, by weight of the total of(a) and (b) solids, of lithium polysilicate containing about 15% throughabout 35%, by weight, of silica as SiO having an SiO /Li O mol ratio of4-25/1, and being free of alkali metal ions other than lithium;

(b) about 25 through about by weight of the total of (a) and (b) solids,of a particulate polymer of ethylenically unsaturated hydrocarbonmonomers completely substituted with fluorine atoms or a combination offluorine atoms and chlorine atoms; and

(c) a liquid carrier; overlaid with a film of fluorocarbon polymer.

2. An article according to claim 1, wherein the composition containsabout 15% through about 40% of lithium polysilicate and about 60%through about 85% of a fluorocarbon polymer.

3. An article according to claim 2, wherein the lithium polysilicate hasa SiO L mol ratio of 4--10/ l.

4. An article according to claim 2, wherein the lithium polysilicate inthe composition has a SiO /Li O mol ratio of 8-9/1.

5. An article according to claim 2, wherein the fluorocarbon polymer in(b) in polytetrafluoroethylene.

6. An article according to claim 1, wherein the composition containsabout 25% of lithium polysilicate and about 75% of a fluorocarbonpolymer.

7. An article according to claim 6 wherein the lithium polysilicate inthe composition has a SiO /Li O mol ratio 8. An article according toclaim 6, wherein the lithium polysilicate in the composition has a SiO/Li O mol ratio of 89/ 1.

9. An article according to claim 6, wherein the fluorocarbon polymer in(b) is polyetetrafiuoroethylene.

10. An article according to claim 1, wherein the lithium polysilicate inthe composition has a SiO /Li O mol ratio of 4-10/1.

11. An article according to claim 1, wherein the lithium polysilicate inthe composition has a SiO /Li O mol ratio of 8-9/1.

12. An article according to claim 1 wherein the fluorocarbon polymer ispolytetrafluoroethylene.

References Cited UNITED STATES PATENTS Hochberg 117-132 CF X Huntsberger117--232 Sears 106-84 Sanders 11775 X Taylor 117161 UF X 6 FOREIGNPATENTS 587,496 11/1959 Canada 260296 F RALPH HUSACK, Primary ExaminerUS. Cl. X.-R.

11749, 75,124 E, 128.4,132 CF, 135.1

